This invention relates to a catenary anchor leg mooring buoy of lightweight construction.
Conventional Catenary Anchor Leg Mooring (CALM) Buoys are used extensively to facilitate offshore marine terminals for the import or export of crude oil (and other fluids).
The CALM buoy is generally moored to a 4, 6 or 8 point mooring system (dependent upon operational requirements, environmental conditions, water depth etc). The tanker to be loaded (or offloaded) moors directly to the buoy and connects to a floating hose which carries the fluid product.
The fluid hose and the mooring assembly arrangements on the CALM buoy are swivel mounted in order that the tanker can xe2x80x9cweathervanexe2x80x9d around the buoy thus maintaining a heading into the wind/weather at all times.
The use of a CALM buoy provides a low cost marine terminal as dedicated port or harbour facilities are not required for tanker mooring. The CALM buoy design allows the tanker to moor directly to the buoy and the fluid transfer pipeline may run from the shore facility to the CALM buoy.
Conventional CALM buoys have been in use in the offshore industry for many years and have proved to be a cost efficient method for the transfer of petroleum products from a seabed production facility to an offtake tanker (or vice versa).
Conventional CALM buoys generally take the form of a large steel cylinder with a central opening or so-called xe2x80x9cmoonpoolxe2x80x9d and a rotating turntable or arm section fitted to the top of the buoy. Generally the turntable is made up of three xe2x80x9carmsxe2x80x9d, the mooring arm, the offtake arm and a counterbalance arm.
The tanker is moored to the mooring arm of the turntable via a hawser mooring system. The tanker is free to weathervane around the buoy by the mooring loads applying rotational forces to the turntable. The turntable is fitted to the main body of the buoy via a large slew bearing arrangement.
A riser/hose system is connected from the seabed facility to a fluid swivel located in the centre of the moonpool. The output flange of the fluid swivel is connected to a pipe that is fixed on to the turntable""s offtake arm and leads off the buoy to an offtake hose connected to the tanker.
The main body of the buoy is generally moored to the seabed via four, six or eight mooring lines. The method used to tension and attach the mooring lines to the buoy differs between designs, but generally the lines are tensioned with an on board winch and gantry arrangement also used for attaching the mooring hawser and offtake hoses. The mooring lines are then locked in to place by a locking device such as a chain stopper. The components of the mooring system are dependent on the water depth, the environment associated with the location where the buoy is to be moored, and the size of the offtake tanker.
Conventional CALM buoys are generally constructed of a steel plate hull divided into several tanks, with a central moonpool and a steel turntable. Due to the steel plate construction the buoys generally weigh in excess of 150 tonnes and can have an overall diameter of up to 15 m.
These conventional CALM buoys are utilised within the offshore oil industry as a cost effective solution for long term offshore projects. However, for short term projects such as well tests and extended well tests they have certain disadvantages.
There are three main areas which render current CALM buoy designs unsuitable for short term projects, these are capital cost, transportation and installation costs and maintenance and repair costs.
The initial purchase cost of a conventional CALM buoy design is high due to the complex construction techniques involved, the quantity of steel required, the requirement for a dockyard/steelworking facilities and the time required.
Due to the size and weight of conventional CALM buoys and their mooring systems the transportation and installation can be time consuming and expensive especially if the final location is distant.
The maintenance and repair of conventional CALM buoys can be expensive due to their size. There is a great deal of steelwork to be surveyed, thickness measurements taken and paint work to be checked. Should one of the hull compartments be breached then the buoy has to be removed and dry-docked. Should the slew bearing suffer damage the buoy generally requires to be removed to a shore facility for repair. Production time for the marine terminal will be lost while a replacement arrangement is installed.
For a long term project such as oil production/export systems the costs associated with conventional CALM buoys are generally acceptable and cost effective when compared to the use of a fixed installation or the provision of port facilities.
For a short term project such as a well test or an extended well test in a remote area costs can be prohibitive.
It is an object of the present invention to provide a lightweight CALM buoy design which can be easily and quickly constructed and which can be maintained and repaired in a cost effective manner. The lightweight CALM buoy design is intended to be a cost effective solution for short term projects which require a tanker mooring facility and the transfer of fluids offshore.
U.S. Pat. No. 4,449,946 describes a mooring buoy of modular construction composed of an open framework mounting buoyancy modules which act as fenders.
According to the present invention and as is known from U.S Pat. No. 4,449,946 and as is known from U.S. Pat. No. 1,449,946 there is provided a catenary anchor leg mooring (CALM) buoy of modular construction and comprising a main framework, at least one replaceable buoyancy module mounted to the main famework, mooring means and fluid application means rotatably mounted to said main framework and serving for connection to an offshore structure, such as a tanker, and means for attachment to at least one mooring line for mooring the buoy.
In contrast to U.S. Pat. No. 4,449,946 and in accordance with the invention the buoyancy module is held wholly within the framework so as to be protected against collision damage and the attachment means for the at least one mooring line is provided on the framework. Accordingly the framework is anchored by the mooring line.
Several detachably mounted modules can be symmetrically mounted to the main framework. Preferably the main framework comprises several elongate frame members which may be structural sections such as tubulars and can be steel.
Preferably the or each buoyancy module is a conventional elastomer material buoy.
More preferably six or twelve of said buoyancy modules are provided, one or two respectively being positioned in each of six segments of the main framework of hexagonal configuration in plan view.
Internal framework may be used within the main framework to mount the buoyancy modules.
Preferably also the mooring means and fluid offtake means are provided on a single arm assembly rotatably mounted on the main framework. The arm assembly then has two arm sectionsxe2x80x94a mooring arm section and a counter-balance arm section.
The mooring lines for mooring the buoy may be three lines secured at 120xc2x0 intervals or, if extra security is required, six lines.
Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: